Count all coprime with N in range [1,N] (Euler's totient function)

euler.cpp

#include <bits/stdc++.h>

using namespace std;
typedef long long ll;

ll phi(ll n){
    ll res = n;
    for(ll i = 2; i * i <= n; i++){
        if(n % i == 0){
            while(n % i == 0) n /= i;
            res -= res / i;
        }
    }
    if(n != 1) res -= res / n;
    return res;
}

int main(){
    ll n;
    cin >> n;
    cout << phi(n);
}